MIR: improve documentation

Context

Document most* public functions, types, enum variants, etc. Add some simple examples.

This also does some minor reshuffling for some semblance of consistency and marks some modules that only define implementations private to avoid cluttering the docs.

* documentation omitted on enum variants and types where meaning is self-evident.

Manually testing the MR

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR